Demand for secondary batteries as an energy source has been significantly increased as technology development and demand with respect to mobile devices have increased. Among these secondary batteries, lithium secondary batteries having high energy density, high voltage, long cycle life, and low self-discharging rate have been commercialized and widely used.
In a lithium secondary battery, a positive electrode is prepared by coating a positive electrode collector with a composition for forming a positive electrode, which is prepared by collectively mixing a positive electrode active material, a conductive agent, and a binder in a solvent, and then drying the coated positive electrode collector.
However, since the positive electrode active material and the conductive agent are typically used in the form of powder, miscibility with the solvent is low when the positive electrode active material and the conductive agent are collectively added to the solvent and mixed, and thus, the positive electrode active material and the conductive agent may be non-uniformly dispersed in the composition. In a case in which a positive electrode active material layer is formed by coating the positive electrode collector with the composition for forming a positive electrode in which components, such as the positive electrode active material and the conductive agent, are non-uniformly dispersed as described above, uniform coating on the positive electrode collector is difficult, and, as a result, the positive electrode active material layer having low thickness uniformity or surface defects is formed to reduce battery performance and lifetime characteristics.
Also, with respect to the conductive agent used in the preparation of the composition for forming a positive electrode, since the conductive agent is typically fine particles of a few tens of nanometers (nm), cohesion is strong, and thus, agglomeration of the fine conductive agent particles may easily occur when the conductive agent particles are dispersed in the solvent. Accordingly, the non-uniform dispersion of the conductive agent in the composition for forming a positive electrode is facilitated, and as a result, conductivity in the positive electrode active material layer becomes insufficient.
Furthermore, in a case in which non-uniform dispersion of the binder in the composition for forming a positive electrode occurs, non-uniformity in adhesive strength of the positive electrode active material layer with respect to the positive electrode collector may occur during the preparation of the positive electrode, and as a result, the positive electrode active material layer may be separated from the positive electrode collector and exfoliated. In this case, the exfoliation may not only significantly reduce the battery performance itself, but also may be a cause of reducing the lifetime characteristics of the battery.
Thus, uniform dispersion of the components in the composition for forming a positive electrode as well as the development of battery components having excellent characteristics is important to improve the battery performance and lifetime characteristics.